Household electricity alternates in polarity sixty times per second (fifty in some countries). Thirty times per second, the positive pole changes to the negative pole, then changes back. Why? The first commercial electricity was supplied as direct current (DC), in which each electrical pole is constant. This worked fine for short distances at low power, but it was impractical to send DC power very far from the generating station. Efficient long-line transmission requires high voltage, because less energy is lost that way. Transforming DC power (changing its voltage) is difficult. Since alternating current is easy to convert to different voltages using simple coil transformers, it was chosen for its flexibility. Alternating current was invented by Nikola Tesla.